1. Field of the Invention
This invention relates generally to the field of flame arrestors in pipe line applications.
2. Background of the Invention
A detonation flame arrestor is designed to extinguish a flame front resulting from an explosion or detonation of the gas in the line. However, in addition to extinguishing the flame, the flame arrestor must be capable of dissipating (attenuate) the pressure front that precedes the flame front. The pressure front (shock wave) is associated with the propagation of the flame front through the unburnt gas toward the flame arrestor. The flame induced pressure front is always in the same direction as the impinging flame travel. The pressure rise can range from a small fraction to more than 100 times the initial absolute pressure in the system.
A flame arrestor apparatus usually comprises flame extinguishing plates, ribbon and/or some type of fill media which includes very small gaps of a small diameter, typically less than the MESG of gases, media with passages that permit gas flow but prevent flame transmission by extinguishing combustion. This results from the transfer of heat from the flame to the plates and/or fill media which effectively provides a substantial heat sink.
Two very common flame arrestor element designs are of a crimped ribbon type such as described in U.S. Pat. Nos. 4,909,730, 5,415,233 as well as parallel plate type as described in U.S. Pat. No. 5,336,083 and Canadian Patent No. 1,057,187. The above is referred to as straight path flame arrestors because the gas flow takes a straight path from the channel entrance to the exit.
Flame arrestors are often used in installations where large volumes of gas must be vented with minimal back pressure on the system. It is generally understood that even small deviations in channel dimensions can compromise flame arrestor performance.
A known conflict results from the fact that gas line pressure is frequently maintained at atmospheric pressure or higher. Pressure drop resulting from a flame arrestor or back pressure created as a result of gas passage through the flame arrestor are undesirable. However, pressure drop resulting from passage of the flame through the plates, ribbons, or fill media in the flame arrestor assists in effectively extinguishing the flame. As a result, a need, therefore, exists for a detonation flame arrestor design which includes a large pressure drop per unit volume but a small aggregate pressure drop over the entire apparatus.
The extinguishing process (flame arrestment) is based on the drastic temperature difference between the flame and fill media material. As such, this is a process that not only depends on the temperature gradient, but also on the hydraulic diameter of the passages and the thermal conduction properties of the gas and the fill media.
The level of turbulence significantly affects the rate of heat loss of the flame within the flame arrestor passages. Turbulence is desirable to facilitate the level of heat loss within the flame arrestor. However, straight path flame arrestors of the currently known designs are inefficient in maximizing the amount of turbulence for effective flame arrestment. This is partly because the path of the flame front is unaltered through the flame arrestor. In addition, known straight path flame arrestor designs are inefficient in dispensing the initial shock wave or reflective shock wave. A need exists for a flame arrestor design which alters the flow of the flame front as it passes through the flame arrestor.
In addition, the fill media commonly used for detonation flame arrestors commonly include ceramic beads. Although ceramic beads have useful thermal characteristics, they are relatively fragile and cannot be compacted without crushing to minimize the space between adjacent beads, thereby maximizing surface area of the fill media and varying the path of travel of the flame creating additional turbulence. The ceramic media could also be crushed by the shock wave thereby leaving gaps larger than the MESG of the gas which would compromise the performance (flame stopping capabilities) of the flame arrestor. A need, therefore, exists for a flame arrestor including a fill media which can be compacted to minimize air space and surface area, thereby maximizing the heat sink properties of the fill media as well as increase turbulent flow through the spaces between adjacent components of the fill media.
A detonation flame arrestor must also be capable of attenuating a reflective pressure front in addition to the initial pressure front (shock wave). Initial shock waves impacting flame arrestor elements have been known to cause significant structural damage (element breach) causing the flame arrestor element to fail.
Prior art devices have been known to fail due to the pressures encountered in connection with a reflection pressure front. Although the flame is extinguished within the flame arrestor, a high pressure wave front may exit the outlet side of the flame arrestor as a result of the pressure rise from the initial shock wave. This high pressure wave front continues to travel along the pipe line in the direction of flow. This high pressure wave front, however, will be reflected by any discontinuity located in the pipe line. Discontinuities are the result of bends, stubs, valves, reducers, and the like. As a wave front strikes such a discontinuity, a reflection front is created which travels back towards the flame arrestor. Reflections from many objects along a pipe line can cause transient pressure increases many times the initial pressure. When these reflections enter the outlet side of the flame arrestor, the pressure within the flame arrestor can become many times that for which it was designed. While these pressure increases are of extremely short duration and transient in nature, they nonetheless are known to cause failures in flame arrestors.
A need, therefore, also exists for a flame arrestor that includes the capability of attenuating an initial shock wave and a reflection pressure front.
Another important factor in flame arrestor design relates to cleanability. Presently known parallel plate, ribbon, and/or fill media designs are known to become blocked or clogged as a result of collection of contaminant particles carried in the gas stream. Once significant clogging occurs which restricts flow and increases pressure drop, the entire flame arrestor must be removed for cleaning or replacement. A need exists for a flame arrestor design which can be cleaned in stream and/or easily accessed for cleaning and/or replacement of the fill media.
Detonation flame arrestors known presently in industrial applications are not known to be effective for low Maximum Experimental Space Gap (MESG) gases, such as Group B gases. In particular, known detonation flame arrestors are not effective for hydrogen gas or enriched oxygen and hydrogen applications. Ribbon or parallel plate detonation flame arrestor constructions cannot be cost effectively produced to meet the requirements of low MESG applications. A need, therefore, exists for a detonation flame arrestor design which can be manufactured in a cost effective manner which is capable of operation in low MESG gas environments.
The detonation flame arrestor of the present invention includes, generally, an outer member or cylinder secured to a canister flange, an inner member or cylinder secured to the canister flange and a fill media retained between the outer and inner cylinders. Both the outer cylinder and inner cylinder, while being secured to the canister flange on one end, include a domed face on their other end. The outer cylinder, inner cylinder, and canister flange together form a canister. The canister is secured within an outer housing bolted to a bulkhead which is welded to the inside of the outer housing. The outer housing is then fitted in the pipeline flow path such that the flow of gas passes into the outer housing and through the canister.
Both the outer cylinder and the inner cylinder include a spiral wound wedge wire screen which form their respective cylindrical circumferences. The respective spiral wound wedge wire screens of both the outer cylinder and the inner cylinder include wound wire having a tapered surface and a blunt (flat) surface such that the direction of the taper on the outer cylinder circumference points in the direction of flow of gas in the pipeline while the tapered surface of the inner cylinder points in the direction of flow of the gas in the pipeline, (pointing against a reverse flow). The inner cylinder is of a smaller diameter than the outer cylinder such that when the canister is assembled, the inner cylinder fits inside the outer cylinder such that the fill media is retained between the flat surface of the spiral wound wedge wire screen of the outer cylinder and the flat surface of the spiral wound wedge wire screen of the inner cylinder.
The domed face of the outer cylinder includes a hole to receive a media displacing bolt. The hole may be drilled and tapped so that the media displacing bolt may be threaded into the hole to accommodate tightening or removal. If a permanent canister construction is desired, the media displacing bolt may be welded in the hole in the domed face of the outer cylinder. The media displacing bolt is tapered such that when threaded through (or inserted and welded) the domed face of the outer cylinder, the tapered portion of the media displacing bolt presses into the fill media thereby compacting the fill media so as to reduce the air space between adjacent elements of the fill media.
The canister is positioned within the outer housing such that a pressure front which passes through the pipeline and into the outer housing impinges upon the domed face of the outer cylinder and the bulkhead. The detonation wave front is attenuated by the domed face of the outer cylinder and the bulkhead. Likewise, after the flame front is extinguished by passage through the canister, a reflected pressure front will impinge the underside of the domed face of the inner cylinder and become attenuated.
After the flame front impacts the domed face of the outer cylinder, it must make an abrupt (ninety degree (90xc2x0)) turn in order to pass through the spiral wound wedge wire screen of the outer cylinder. The gap size between adjacent windings of the spiral wound wedge wire screen can be chosen for a particular gas or gas group and acts as the first mechanism for arresting the flame passing therethrough. The flame then passes through the fill media and is further quenched as a result of passing through the torturous path required to pass through the fill media and contacting the surface of the fill media (heat sink). Once the quenched gas exits the fill media, it passes through the spiral wound wedge wire screen of the inner cylinder which is likewise gapped for a chosen gas or gas group. Once the gas exits the inner cylinder, it must again make an abrupt (ninety degree (90xc2x0)) turn to continue flow through the pipeline.
Accordingly, flame arrestment is achieved in the detonation flame arrestor of the present invention through the combination of the gaps between adjacent windings of the spiral wound wedge wire screens on both the outer cylinder and inner cylinder as well as the irregular shaped fill media. The gap size between adjacent windings of the spiral wound wedge wire screen being lower than the MESG of the gas so as to provide the first mechanism for flame arrestment. The irregular shaped fill media provides a torturous flame path and large heat transfer area between the flame front and the fill media.
This transverse design of the flame arrestor of the present invention serves two very significant functions. First, it allows the shock wave to impact the high strength surfaces of the domed faces of the outer cylinder and the bulkhead as stated above. The second function is to allow the total surface area (dictated by the length) of the canister to be varied to accommodate a desired pressure drop simply by lengthening the canister as opposed to increasing the diameter as with a straight path design.
In the preferred embodiment, the fill media consists of irregular shaped spheres such as cut-wire shot. The irregular shaped spheres create irregular sized gaps between adjacent compacted spheres in the fill media. The irregular shape of the individual components of the fill media as well as the irregular shaped gaps formed between adjacent spheres disrupts the laminar flow of a flame wave (creates turbulence). Moreover, in addition to increasing turbulence, the fact that the spheres are of irregular shape means that they have greater surface area than precision spheres to create a heat sink so as to extinguish a flame passing therethrough. Accordingly, increased heat transfer is achieved. The canister, including the fill media contained therein, is designed to provide an optimum pressure drop per unite volume to provide maximum flame arrestment. Again, as a result of the transverse design, the aggregate pressure drop resulting from the passage of the gas through the canister can be maintained at a low value by varying the length of the canister as required.
The tapered surface of the wire forming the spiral wound wedge wire screen serves the dual purposes of providing aerodynamic gas flow characteristics into the canister and also to provide a tapered or angled surface such that debris is trapped between adjacent windings of the tapered surface of the spiral wound wedge wire screen. Aerodynamic gas flow is created by the point of the taper cutting through the gas flowing past. Allowing the gas to flow past improves the flow characteristics without causing a significant pressure drop. In addition, while a parallel plate design would contribute to laminar flow of the gas cutting through the plates, the tapered wedge wire, in contrast, contributes to increase turbulence by increasing velocity and decreasing pressure of the shock wave.
Debris trapped between adjacent windings of the tapered surface of the spiral wound wedge wire screen can be easily dislodged upon a reverse flow within the canister by injecting a high pressure cleaning solution through the domed face of the outer cylinder of the canister. This can be accomplished by installing high pressure nozzles in the domed face of the outer cylinder adjacent the media displacing bolt.
The size of the gaps between adjacent windings of the spiral wound wedge wire screen of both the outer cylinder and the inner cylinder acts to extinguish a flame passing therethrough according to known characteristics of selected gases. Thus, a gap size can be selected depending upon the type of gas to be carried by the application, and secondarily, the wound wedge wire screen also serves to contain the fill media.
The wedge wire screen on the inner and outer cylinders can be effectively produced by spiral winding a tapered wire around their respective cylindrical circumferences. The gap size can be controlled so as to be lower than the published (known) MESG properties of a particular gas or gas group winding the tapered wire around the cylinders can be done economically while maintaining strict tolerances. The design of the present invention is therefor, effective for low MESG gas applications, such hydrogen.
The fill media can be recharged or replaced by removing the canister from the external housing, removing the fill media by removing the tapered displacing bolt, and replacing the fill media with fresh fill media. The new fill media could be of a different size as required with a different size to accommodate a different gas, type, or group, as desired. Alternatively, the removed fill media can be cleaned and reinstalled for continued use.
It is therefore an object of the present invention to provide a detonation flame arrestor that includes a canister which requires the flame front to make an abrupt direction change to pass through the canister.
It is an additional object of the present invention to provide a detonation flame arrestor which includes a spiral wound wedge wire screen.
It is a further object of the present invention to create a detonation flame arrestor including a spiral wound wedge wire screen on an inner cylinder and an outer cylinder together forming the canister.
It is yet a further object of the present invention to provide a detonation flame arrestor including a spiral wound wedge wire screen using a wire which is tapered on at least one surface so as to trap debris and increase the flow and create turbulence characteristics through the wedge wire screen.
It is a still further object of the present invention to provide a detonation flame arrestor including a spiral wound wedge wire screen which also includes a gap between adjacent windings of the screen selected for a particular gas type or gas group.
It is yet an additional object of the present invention to include a fill media between the inner cylinder and outer cylinder to act as a torturous path and heat sink to extinguish a flame passing therethrough.
It is a yet another object of the present invention to include an irregular shaped fill media to increase surface area and also to increase the turbulence of the gas/flame passing therethrough.
It is an object of the present invention to provide a detonation flame arrestor design which is effective for low MESG gas applications.
It is also an object of the present invention to provide a detonation flame arrestor including an inner cylinder and outer cylinder with a fill media therebetween which is capable of being removed for cleaning/recharge or replaced with a fill media of a different, size/characteristic selected for a different gas type or gas group.
Additional objects of the present invention include attenuation of the pressure front and reflective pressure front by designing the flame arrestor to provide a structurally sound domed face on both the outer cylinder and inner cylinder.
Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following description of the preferred embodiments.